If the policy and decision makers are a single entity then I'd write it like this...
This study benefits vendors, and policy and decision makers
If they are two separate entities then its clearer to write it like this using an Oxford comma...
This study benefits vendors, policy makers, and decision makers
The Oxford comma can be used as the final comma in a list of 3 or more things so that the reader doesn't confuse the last two items in the list as being combined.
This is a good example I found on Grammarly.com:
"I love my parents, Lady Gaga and Humpty Dumpty"
Without the Oxford comma, the sentence above could be interpreted as
stating that you love your parents, and your parents are Lady Gaga and
Humpty Dumpty. Here’s the same sentence with the Oxford comma:
"I love my parents, Lady Gaga, and Humpty Dumpty"